Electric double layer capacitor

ABSTRACT

An electric double layer capacitor having an element impregnated with a non-aqueous electrolyte, the element comprising positive and negative electrodes made of carbonaceous electrodes, and a separator interposed between the electrodes, wherein the separator is made of paper prepared to contain at least 50 wt % of fibers obtained by beating regenerated cellulose fibers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-aqueous type electric doublelayer capacitor which has a high power output and a high energy density.

2. Discussion of Background

Heretofore, as a separator to be disposed between positive and negativeelectrodes of an electric double layer capacitor, a polyethylenenon-woven fabric, a polypropylene non-woven fabric, a polyesternon-woven fabric, kraft paper, a rayon/sisal mixed sheet, a manila hempsheet or a glass fiber sheet is, for example, known (e.g. JP-A-9-45586,JP-A-1-304719). The role of a separator is to electrically insulate thepositive electrode from the negative electrode on one hand and tofacilitate transfer of ions in the electrolyte, which takes placeaccompanying charge and discharge, on the other hand.

In recent years, an attention has been drawn to an electric double layercapacitor for high power applications. However, with a separator made oforganic fibers e.g. a polyethylene, the ion conductivity is low, and theinternal resistance of the electric double layer capacitor is high,since the liquid absorbing property and the liquid-holding property forthe electrolyte are low. Accordingly, if instantaneous large currentdischarge was carried out as one of main characteristics of an electricdouble layer capacitor, the voltage drop was substantial, such being notpractical.

Further, a conventional separator made of paper is excellent in heatresistance and tensile strength and is sometimes effective for anelectric double layer capacitor to be used for a power source wherein nolarge current discharge is carried out, like a hybrid power source usedwith a solar cell. However, when a conventional separator made of paperis used for an electric double layer capacitor for high powerapplications, wherein the electrolyte is non-aqueous, the ionpermeability tends to be inadequate.

With an electric double layer capacitor, it is desired to lower theresistance and to increase the capacitance per unit volume, and it isaccordingly required to make the separator as thin as possible. However,if a separator made of paper is made thin, the insulating propertybetween the positive and negative electrodes tends to be inadequate,thus leading to micro-short circuiting, serious self-discharge.

SUMMARY OF THE INVENTION

Under these circumstances, it is an object of the present invention torealize reduction of the resistance and increase of the capacitance foran electric double layer capacitor by employing a separator which isexcellent in heat-resistance and ion permeability, even if it is thin,and has high strength and excellent electronic insulating properties.

The present invention provides an electric double layer capacitor havingan element impregnated with a non-aqueous electrolyte, said elementcomprising positive and negative electrodes made of carbonaceouselectrodes, and a separator interposed between the electrodes, whereinsaid separator is a paper prepared to contain at least 50 wt % of fibersobtained by beating regenerated cellulose fibers.

Now, the present invention will be described in further detail withreference to the preferred embodiments.

In the present invention, the starting material of the separator for theelectric double layer capacitor using a non-aqueous electrolyte, isregenerated cellulose fibers which can be beaten with a beating machineinstalled in a usual paper-making process, such as regenerated cellulosefibers having a high polymerization degree, or solvent-spun rayonfibers. They are used after beaten for the material of the separator.

If the paper constituting the separator is made of less than 50 wt % offibers obtained by beating regenerated cellulose fibers, the separatorobtained therefrom tend to have high electric resistance and poorstrength. In order to maintain the strength of the separator, it isconceivable to increase its thickness. However, it is no good since suchmethod increases high electric resistance of the separator. In thepresent invention, the paper constituting the separator is made ofpreferably at least 65 wt %, particularly preferably at least 80 wt %,of fibers obtained by beating regenerated cellulose fibers.

Regenerated cellulose fibers will be uniformly fibrillated and will haveincreased flexibility with the beating treatment. Accordingly, theseparator made of paper prepared to contain at least 50 wt % of suchfibers, is excellent in tensile strength. Further, the fibersfibrillated by beating, are extremely dense, and the cross sections offibrils are substantially circular. Accordingly, the separator made ofpaper prepared to contain at least 50 wt % of such fibers, has a lowresistance.

The regenerated cellulose fibers of the present invention are preferablybeaten until from 0 to 600 ml of Canadian Standard Freeness (hereafterreferred as CSF value) stipulated in JIS P8121. The regeneratedcellulose fibers from solvent-spun rayon etc. have a CSF value of about800 ml in the unbeaten state. Unless such fibers are adequately beaten,they will not be sufficiently fibrillated and will have low strength dueto inadequate bonding site obtained by fibrils. Accordingly, such fibersare preferably beaten until the CSF value become 600 ml or less. Theregenerated cellulose fibers may be beaten until the CSF value become 0ml because they are expected to have a high strength as their densityincreases in proportion to the degree of the beating.

Other material to be incorporated to the beaten regenerated cellulosefibers, is not particularly limited, and any fibers such as Manila hemp,sisal or kraft pulp, may be used. Such a material is preferably beatendepending upon the degree of beating of the regenerated cellulosefibers.

The separator in the present invention can be prepared, for example, asfollows. Firstly, regenerated cellulose fibers cut into a few mm arebeaten to have proper CSU value by a beating machine. On the other hand,fibers to be used as a blend material are likewise beaten to a properdegree, followed by mixing so that the regenerated cellulose fibers arecontained in an amount of at least 50 wt %, whereupon a paper havingpredetermined thickness is prepared. The paper thus obtained is used asthe separator which is interposed between positive and negativeelectrodes.

In the present invention, the separator has preferably a thickness offrom 20 to 60 μm and a density of from 0.30 to 0.60 g/cm³. If thethickness is less than 20 μm, inadequate insulation between the positiveand negative electrodes is likely to occur. If the thickness is morethan 60 μm, the energy density tends to be low. Further, if the densityis less than 0.3 g/cm³, the strength of the separator becomesinadequate. If the density is more than 0.60 g/cm³, the ion permeabilitybecome insufficient. It is particularly preferred that the thickness isfrom 30 to 50 μm and the density is from 0.35 g/cm³ to 0.50 g/cm³.

In the present invention, the separator may be made by overlaying sheetsof the paper prepared to contain at least 50 wt % of fibers obtained bybeating regenerated cellulose fibers. Further, the separator may also becomposed by overlaying said paper and paper made of different materialfrom said paper or non-woven fabric. When the separator is composed ofsheets of the paper overlaid, a multilayered paper prepared at aprocedure of paper making can be used. The separator prepared byoverlaying a plurality of said paper or by overlaying the other paperand said paper is preferred in order to avoid short-circuit between thepositive and negative electrodes. However, its power output per unitvolume tends to be low because the thickness of the separator becomelarge.

The separator composed of the paper prepared to contain at least 50 wt %of fibers obtained by beating regenerated cellulose fibers usuallycontains from 3 to 10 wt % of moisture. It is preferred to remove such amoisture in order to reduce the leakage of current and to secure a highwithstand voltage in the non-aqueous type electric double layercapacitor. The moisture contained in said paper for the separator in useis preferably not higher than 1 wt %.

In order to efficiently remove the moisture, it is preferred to heat theseparator at a temperature of from 90 to 250° C. before disposing itbetween a positive electrode and a negative electrode. In order toobtain an electric double layer capacitor having a particularly largecapacitance, it is preferred to have a structure such as a cylindricaltype prepared in such a manner that a pair of electrodes are wound withan separator interposed therebetween, to form an element, and theelement is impregnated with a non-aqueous electrolyte and accommodatedin a bottomed cylindrical casing, or an angular type prepared in such amanner that a plurality of rectangular electrodes as positive andnegative electrodes are alternately laminated with a separatorinterposed therebetween, to form an element and the element isimpregnated with a non-aqueous electrolyte and accommodated in abottomed angular casing. But, it is difficult to remove the moistureefficiently after an element is formed by the electrode and theseparator.

If the temperature of heat treatment is lower than 90° C., inadequatereduction in the leakage of current etc. can be obtained because theremoval of the moisture in the separator is not sufficient. On thecontrary, if the temperature of heat treatment exceeds 250° C., theseparator itself is likely to undergo thermal decomposition, whereby thestrength tends to be low and water may be generated. The temperature forheat treatment is, more preferably, from 120 to 230° C. The time forheat treatment is usually at least 3 seconds, while it is suitablyselected from the relation with the temperature for heat treatment.

The method for the heat treatment may suitably be selected from methodssuch as contacting with a hot heater, irradiation with ultraviolet raysor leaving in an atmosphere of heated air. The separator is usuallyavailable in a rolled state, but in such a rolled state, it is difficultto effectively remove the moisture by heating in a short period of time.It is preferred to heat it in a state not laminated, so that removal ofthe moisture can effectively be carried out. Specifically, a rolledpaper for the separator may be rewound under heating in a dry atmosphereto obtain a dehydrated paper roll, or a plurality of paper sheets arepreliminary cut out from the rolled paper for the separator, and heatedto remove moisture in such a state that the paper sheets are notintimately put together, e.g. by having a heat-resistant porous spacerinterposed therebetween.

In the electric double layer capacitor of the present invention, theelectrodes for both positive and negative electrodes, are carbonaceouselectrodes comprising a carbon material as the main component, and thecapacitor is based on a principle that electric charge is stored in anelectric double layer formed at the interface between the electrodes andthe electrolyte. To increase the capacitance of an electric double layercapacitor, the specific surface area of the carbon material ispreferably large, and the carbonaceous electrodes are preferably made ofa carbon material having a specific surface area of from 700 to 2,500m²/g and an organic binder.

As the carbon material, activated carbon, carbon black or polyacene may,for example, be used. To the carbonaceous electrodes, an electricallyconductive material may be incorporated to increase the electricalconductivity, as the case requires. An organic binder is added to thecarbon material, and the carbonaceous electrode is formed into a sheetshape on a metal current collector so that it is integrated with thecurrent collector to form an electrode assembly. The organic binder tobe used here, may preferably be, for example, a polyvinylidene fluoride,a polytetrafluoroethylene, a polyimide resin or a polyamideimide resin.The metal current collector may, for example, be a foil, a net or thelike of e.g. aluminum or stainless steel. Particularly preferred isaluminum, since it is light in weight and has a low resistance.

The electrolyte to be used for an electric double layer capacitorincludes an aqueous electrolyte and a non-aqueous electrolyte. However,the nominal voltage is about 0.8 V with the aqueous electrolyte, whileit is about 2.5 V with the non-aqueous electrolyte. The stored energy ofan electric double layer capacitor is proportional to the square of thecell voltage. Accordingly, from the viewpoint of the energy density, itis preferred to use the non-aqueous electrolyte, as the energy densitycan be made larger by about 9 times.

The solute for the non-aqueous electrolyte for the electric double layercapacitor of the present invention is preferably at least one saltcomprising a quaternary onium cation represented by R¹R²R³R⁴N⁺orR¹R²R³R⁴P⁺, wherein each of R¹, R², R³ and R⁴ which are independent ofone another, is a C¹⁻⁶ alkyl group, and an anion such as BF₄ ⁻, PF₆ ⁻,SO₃CF₃ ⁻, AsF₆ ⁻, N(SO₂CF₃)² ⁻or ClO₄ ⁻.

Further, the organic solvent to be used for the non-aqueous electrolyteis preferably a cyclic carbonate such as ethylene carbonate, propylenecarbonate or butylene carbonate, a linear carbonate such as dimethylcarbonate, ethyl methyl carbonate or diethyl carbonate, acetonitrile,sulfolane or a sulfolane derivative. It is particularly preferably atleast one member selected from the group consisting of propylenecarbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate,methyl ethyl carbonate, acetonitrile, sulfolane and methylsulfolane.

As the shape of the electric double layer capacitor of the presentinvention, preferred is a cylindrical type prepared in such a mannerthat a pair of elongated electrodes are wound with an elongatedseparator interposed therebetween, to form an element, and the elementis impregnated with a non-aqueous electrolyte and accommodated in abottomed cylindrical casing, or an angular type prepared in such amanner that a plurality of rectangular electrodes as positive andnegative electrodes are alternately stacked with a separator interposedtherebetween, to form an element, and the element is impregnated with anon-aqueous electrolyte and accommodated in a bottomed angular casing,since it is thereby possible to obtain a large capacitance. Inparticular, the separator having a high strength even a small thicknessin the present invention is suitable for the cylindrical type, whereinthe separator as well as the electrodes is needed to have an adequatetensile strength when it is wound.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted to such specific Examples.

EXAMPLE 1

Solvent-spun rayon was beaten until the CSF value became 20 ml, and thisrayon was used to sheet a paper having a thickness of 40 μm and adensity of 0.40 g/cm³.

The paper was used as a separator. Its tensile strength was 0.70kg/cm-width. This separator contained 7 wt % of moisture. This separatorwas cut into a sheet of 5 cm ×7 cm and was preliminarily dried in air at150° C. for 1 hour to remove water to bring the moisture content to nothigher than 1 wt %.

An electrode (electrode area: 24 cm², electrode thickness: 0.15 mm)obtained by forming into a sheet shape a composition comprising 80 wt %of activated carbon having a specific surface area of 1,500 m²/g, 10 wt% of carbon black and 10 wt % of polytetrafluoroethylene as a binder,was bonded via a conductive adhesive to one side of a rectangularaluminum foil having a width of 4 cm, a height of 6 cm and a thicknessof 50 μm and having a lead terminal, followed by heating to thermallycure the adhesive and further by drying at 200° C. to obtain anelectrode assembly.

In a glove compartment with a dew point of 50° C., two such electrodeassemblies were overlaid to one another with the above-mentioneddehydrated separator interposed, so that the electrode sides faced eachother.

This laminate was sandwiched between a pair of glass plates having athickness of 2 mm, a width of 5 mm and a height of 7 cm, to form acapacitor element. The total thickness of the electrode assemblies andthe separator was 0.44 mm. Then, this element was heated in vacuum at200° C. for 3 hours to further remove impurities and moisture inelectrodes.

As an electrolyte, a solution containing 1.5 mol/l oftriethylmonomethylammonium tetrafluoroborate dissolved in propylenecarbonate, was employed. This electrolyte was vacuum-impregnated to theabove element, and the element was used to obtain an electric doublelayer capacitor. The internal resistance and the capacitance wereobtained at current density of 20 mA/cm². The internal resistance was0.2 Ωn and the capacitance was 13.2 F. The leakage current at a voltageof 2.5 V was 8 μA. The capacitance per 1 cm³ of the element was 12.5 Fand the internal resistance per 1 cm³ of the element was 0.21 Ω.

EXAMPLE 2

70 wt % of fibers obtained by beating Solvent-spun rayon until the CSFvalue became 100 ml and 30 wt % of fibers obtained by beating Manilahemp until the CSF value became 400 ml were mixed and sheeted to obtaina paper having a thickness of 50 μm and a density of 0.35 g/cm³. Acapacitor element was assembled in the same manner as in Example 1except that the sheeted paper was used as a separator, which waspreliminarily dried at 200° C. for 1 hour. The tensile strength of theseparator was 0.96 kg/cm width.

An electric double layer capacitor cell was prepared in the same manneras in Example 1 except that the above element was used, and theperformance was evaluated. The total thickness of the electrodes and theseparator was 0.45 mm. The internal resistance was 0.24 Ω and thecapacitance was 12.1 F. The leakage current at a voltage of 2.5 V was 6μA. The capacitance per 1 cm³ of the element was 11.2 F and the internalresistance per 1 cm³ of the element was 0.26 Ω.

EXAMPLE 3

An electric double layer capacitor cell was prepared in the same manneras in example 1 except that no preliminary drying of the separator wascarried out, and the performance was evaluated. The total thickness ofthe electrodes and the separator was 0.45 mm. The internal resistancewas 0.24 Ω and the capacitance was 12.1 F. The leakage current at thevoltage of 2.5 V was 13 μA. The capacitance per 1 cm³ of the element was11.2 F and the internal resistance per 1 cm³ of the element was 0.26 Ω.

EXAMPLE 4 (Comparative Example)

70 wt % of fibers unbeaten Solvent-spun rayon (the CSF value 800 ml) and30 wt % of fibers obtained by beating Manila hemp until the CSF valuebecame 200 ml were mixed and sheeted to obtain a paper having athickness of 60 μm and a density of 0.35 g/cm³. The paper was used as aseparator, which had a tensile strength of 0.83 kg/cm.

An electric double layer capacitor cell was prepared in the same manneras in Example 1 except that the above element was used, and theperformance was evaluated. The total thickness of the electrodes and theseparator was 0.46 mm. The internal resistance was 1.30 Ω and thecapacitance was 8.2 F. The leakage current at a voltage of 2.5 V was 5μA. The capacitance per 1 cm³ of the element was 7.4 F and the internalresistance per 1 cm³ of the element was 1.44 Ω.

EXAMPLE 5 (Comparative Example)

As a separator was used a sheet of non woven fabric made ofpolypropylene (thickness: 160 μm, weight: 52 g/m²). The separator had atensile strength of 1.53 kg/cm width. An electric double layer capacitorcell was prepared in the same manner as in Example 1 except that thecapacitor element was heated in vacuum at 120° C., and thecharacteristics was evaluated. The internal resistance was 2.0 Ω and thecapacitance was 6.5 F. The leakage current at a voltage of 2.5 V was 14μA. The capacitance per 1 cm³ of the element was 4.85 F and the internalresistance per 1 mm³ of the element was 2.68 Ω.

EXAMPLE 6 (comparative example)

Solvent-spun rayon beaten until the CSF value of 500 ml and with sisalhemp were mixed and sheeted to a paper having a thickness of 70 μm and adensity of 0.48 g/cm³ (a weight ratio of Solvent-spun rayon/sisal hempwas 40/60). A capacitor element was assembled in the same manner as inExample 1 except that the above paper was used as a separator. Thetensile strength of the separator was 1.56 kg/cm width.

An electric double layer capacitor cell was prepared in the same manneras in Example 1 except that the above element was used, and theperformance was evaluated. The total thickness of the electrodes and theseparator was 0.47 mm. The internal resistance was 1.10 Ω and thecapacitance was 9.3 F. The leakage current at a voltage of 2.5 V was 8μA. The capacitance per 1 mm³ of the element was 8.2 F and the internalresistance per 1 mm³ of the element was 1.24 Ω.

According to the present invention, it is possible to obtain an electricdouble layer capacitor which has a low internal resistance, a lowleakage current and a high capacitance density. Further, the separatorof the present invention has adequate strength for winding, andjelly-rolled type electric double layer capacitor can easily beprepared.

The electric double layer capacitor by the present invention isparticularly suitable for a jelly-rolled type or a stacked type electricdouble layer capacitor for a large capacitance or a large current,whereby the discharge capacitance from 50 to 20,000 F or the dischargecurrent from 1 to 1,000 A is attained.

What is claimed is:
 1. An electric double layer capacitor having anelement impregnated with a non-aqueous electrolyte, said elementcomprising positive and negative electrodes made of carbonaceouselectrodes, and a separator interposed between the electrodes, whereinsaid separator is paper prepared to contain at least 50 wt % of fibersobtained by beating regenerated cellulose fibers, wherein thecarbonaceous electrodes are made of a carbon material having a specificsurface area of from 700 to 2,500 m²/g and an organic binder.
 2. Theelectric double layer capacitor according to claim 1, wherein the fibersobtained by beating regenerated cellulose fibers have from 0 to 600 mlof Canadian Standard Freeness stipulated in JIS P8121.
 3. The electricdouble layer capacitor according to claim 1, wherein the separator has athickness of from 20 to 60 μm and a density of from 0.30 to 0.60 g/cm³.4. The electric double layer capacitor according to claim 1, wherein thecarbonaceous electrodes are electrodes obtained by mixing carbonmaternal with an organic binder, followed by forming the mixture into asheet shape.
 5. The electric double layer capacitor according to claim1, wherein the solute of the non-aqueous electrolyte is a saltcomprising a quaternary onium cation represented by R¹R²R³R⁴N⁺ orR¹R²R³R⁴P⁺, eherein each of R¹, R², R³ and R⁴ which are independent ofeach other, is a C₁₋₆. alkyl group, and an anion of BF₄ ⁻, PF₆ ⁻, SO₃CF₃⁻, AsF₆ ⁻, N(SO₂CF₃)₂ ⁻ or ClO₄ ⁻, and the solvent of the non-aqueouselectrolyte is at least one member selected from the group consisting ofpropylene carbonate, ethylene carbonate, dimethyl carbonate, methylethyl carbonate, acetonitrile, sulfolane or methylsulfolane.